Since polyimide resins are excellent in heat resistance, they are widely used for fields of semiconductor elements. In particular, the polyimide resins are used as interlayer insulation films, and surface protective films (buffer coatings) provided between a sealant and a semiconductor chip.
Here, the surface protective film (buffer coating) is a film which plays a role to prevent that an aluminium thin film circuit or an oxidized film formed in a preceding step is damaged while at work of a post-step and cracks occur due to a difference of thermal expansion coefficients of the sealant and silicon after mounting the semiconductor chip. The surface protective film composed of the polyimide resin has been noticed because of having not only ability to protect the chip from external damage as described above but also ability to protect from radiation damage due to having a high α-ray blocking ability. Furthermore, the polyimide resin has been also noticed as a tool to perform a patterning process simply and certainly.
Recently, photosensitive polyimide where a photosensitivity is imparted to the polyimide resin by adding or mixing a compound having a photosensitive group to an acid functional group of a polyimide precursor so that the relief pattern can be formed easily by application, exposure and development has been becoming a mainstream in order to shorten a process for producing the semiconductor (e.g., see Patent Documents 1 and 2).
Also recently, the development by an alkaline aqueous solution is increasingly required in the light of material cost and environmental conservation. Thus currently, a photosensitive resin using a polybenzoxazole precursor having the acid functional group as a base resin or the like have been proposed as the interlayer insulation film and the surface protective film (buffer coating) (e.g., see Patent Document 3).
However, conventional polyimide-based resin has had a problem that it has a corrosive nature to metals such as copper and copper alloys commonly used as semiconductor materials. This is because a carboxyl group which is the acid functional group contained in a structure of polyamic acid in the polyimide precursor reacts with the metal such as copper and copper alloys.
That is, for example, when an interlayer insulation film for a multilayer wiring plate is formed by applying a polyimide precursor varnish on a substrate where a metal wiring or a metal layer composed of the copper or copper alloy has been formed, the carboxyl group contained in the structure in polyamic acid contained in this polyimide precursor varnish causes a corrosive action upon the copper or copper alloy and further generates copper ion. As a result, various problems such as poor insulation, breaking of wire, short circuit, rust in metal portions, reduction of film adhesiveness and deterioration of film physical properties are caused.
Also, when a resist film is formed using a photosensitive polyimide precursor on the substrate where the metal wiring or the metal layer composed of the copper or copper alloy has been formed, the carboxyl group in the photosensitive polyimide precursor chemically reacts with the copper or copper alloy, thereby incompletely removing a polyimide film in an unexposed portion to leave a residual film upon development. In order to solve this problem, it has been proposed to contain 1H-tetrazole or a derivative thereof in the polyimide-based resin (see patent Document 4). However, the adhesiveness, particularly the adhesiveness under a strict condition such as heating and applying pressure is not ameliorated sufficiently even in this method.
The aforementioned polybenzoxazole precursor also causes the corrosive action upon the copper or copper alloy similarly to the polyimide precursor because it also has a phenolic hydroxyl group as the acidic functional group. As a result, various problems, e.g., the poor insulation, the breaking of wire, the short circuit, the rust in metal portions, the reduction of film adhesiveness and the deterioration of film physical properties have been caused.
Patent Document 1: Japanese Patent Application Laid-Open No. 54-109828    Patent Document 2: Japanese Patent Application Laid-Open No. 4-204945    Patent Document 3: Japanese Patent Application Publication No. 1-46862    Patent Document 4: Japanese Patent Application Laid-Open No. 8-286374